There's no way I can do justice to all qualia-related issues, but I'm going to prattle on a bit so that you can get a good handle on what exactly (Tem42 thinks) a qualia is, and how they are used in philosophy.

First up, here's an example of a philosophical issue in which it is useful talk about qualia. It is called the inverted spectrum:

It is possible that when you and I look at a color, we actually percieve different colors. But as long as we consistently label colors, we would not know that one word refers to two different colors. For example, imagine that we were both to look at the same apple; you see the apple as red. But when I see the apple, I see what you would call 'blue'. Since I have always heard everyone call this color 'red', I also call the apple red. Every time I see something blue (to me), I call it by the word 'red', because everyone else does so.

When talking about the inverted spectrum, we are (probably) not questioning whether the apple is reflecting different light waves for me than it is for you. We are questioning whether these light waves are causing the same mental experiences in both of us, that is, if the qualia caused by 'red' light is the same for both you and I.

When talking about such things it is useful, if not necessary, to use some word or term that is equivalent in meaning to qualia.

But wait! Some people will deny that qualia exist! How can this be - there most certainly is some mental experience going on in my mind - how could they deny this? Well, leaving minor (and not so minor) differences between definitions of qualia beside, there are at least two ways that the definition given above could be said to describe something that doesn't exist.

First off, some people will read the definition above and claim that they have never experienced any such thing. Maybe they honestly do not experience this qualia, maybe they are interpreting the definition differently than I am, maybe I'm schizophrenic and seeing things others don't.

Secondly, maybe they're not disputing the definition of 'qualia'. Maybe they're just using a different definition of 'exist'. The everyday definition of 'exist' is probably something along the lines of "look, an x. I guess x's exist". While I am certain that qualia must exist in this sense, there is another sense of exist.

When building an ontology, a person tries to enumerate everything that exists and the forces acting upon them. Since an ontology, if it is complete, should include everything that exists, anything that doesn't appear in this ontology can be said to not exist. This leads to all kinds of weirdness -- for example, if I hold that the complete set of basic building blocks of the universe consists of subatomic particles, I can say that (in my ontology) tables don't exist. That would not mean to imply that we don't see tables in our everyday lives, but it does mean that once I've listed every bit and piece of the universe, I wont have a table listed among those pieces.

Likewise, I could build an ontology without any qualia. Moreover, I can then use my ontology to explain how things like qualia and tables come about. When I do this, I don't ever need to use the word qualia; I might instead use words like 'chemicals' and 'synapses' and 'neurons'. And if I did this, I might decide that having people use the word qualia is only going to complicate their understanding of this universe. To put it another way, mental events might be best understood as completely physical events, not mental, and any mumbo-jumbo suggesting otherwise should be thrown out.

Personally, I am more certain of my qualia than I could possibly be of anything else. I might, for example be a brain in a vat being fed images by a mad scientist (think The Matrix). In this case, this computer that I'm typing at does not exist in any real and objective sense. What does exist is my experience of the computer; the feel of the keys under my fingertips, the glow of the screen, etc. These are the qualia, and they are most certainly here. The existence of qualia is the one thing in the universe that I can be 100% sure of, because they are the only thing that I experience directly.

When I look at and smell a crimson rose and feel its velvety petals touched with dew, I don't detect, or have, any qualia. I don't see qualia, or smell them, or touch them. I see the rose; I see its petals; I see the dew. I do not see crimson qualia, or rose-shaped qualia, or abstract crimson-ness, or a mental act of seeing, or a mental image, or any other such philosopher's entity supposed to be 'mental'. The only things I see are real-world objects several centimetres in front of my face. None of them are in my brain.

They aren't in my 'mind' either, for there are no such things as minds for mental things to take refuge in. Let's not pursue that point here: see Ryle's The Concept of Mind for it.

Similarly when I imagine a rose, or remember a rose, what I imagine or remember is a flower, a physical object. I don't (usually) imagine images, or remember memories. When I smell something, I don't smell my smelling of it, and when I remember something, I don't remember my remembering of it. The thing pointed to by the mental act is a thing in the world. (Not necessarily a real thing, if I'm imagining it, but if I am, I'm imagining a real rose: I don't usually try to imagine imaginary roses, though I could if I was trying to summon up one of impossible size or colour).

We don't see sights, we don’t smell smells, we don’t hear sounds, except in a trivial sense where we see or hear or smell things (clocks, roses, sandalwood) but don’t want to list which exact things. A smell isn’t a kind of thing the way the smell of sandalwood is. It's just a linguistic shorthand. There's no such thing as a 'smell' out there, only specific smells.

We don't see by creating interior mental copies to see. The things we see are objects, not images, abstractions, or even colours. (Locutions like 'seeing red' either are idiomatic, or describe rare physical circumstances where it's not clear what we're seeing.)

The process of seeing doesn't involve mental images. (A mental image is something we can summon up in the absence of an object: we normally don't have them while it's visible.) Seeing isn't a private act, an interior process deep inside of the brain, remote from the rest of the physical chain. None of the following details are individually important to the argument, but, briefly: when you see something, light excites its surface, it emits photons, these travel through the air and reach the eye, this electrochemically stimulates the optic nerve, which forms an image on visual cortex V1, which is then analysed into different aspects as it's fed further into visual cortices, these get mixed with other sensory and motor associations, and these may cause behaviour. This is complex and of course far from fully understood, as yet. Whatever happens, nowhere in this chain are the physical events in the brain converted into qualia or mental copies.

I assert this baldly because there is no purpose that introducing them could serve: the process is described fully without them.

Furthermore, nowhere in this chain is there a single point at which seeing occurs. The whole chain is seeing, including the resultant behaviour. There is no point where seeing enters the mental realm. Possible 'mental' acts resulting from seeing something, such as remembering having seen it, or being surprised at seeing it, are physical processes in the brain like any other, and don't require any additional 'mental' component to prime them. They're caused by neurone firings, not by qualia.

When I feel angry, I don't have a feeling of anger. A 'feeling of anger' is not a kind of thing. It's not something I can have, and it's not something that can populate some kind of mental realm. The situation is fully described by saying that I'm angry, or feel angry. Saying I have it, or that it exists somewhere, is like saying I own it, or that I bought it and took it home and unwrapped it and kept the receipt. None of these are appropriate. A feeling isn't a kind of thing you can have. 'Having a feeling' is just an English idiom, not an iconic representation of a real relationship. You feel certain ways, but you don't feel the feeling (or do anything else to or with the feeling). Feeling angry is some combination of being flushed, speaking loudly, your heart beating faster, being unjust to others, curling your hands into fists, snapping at other people, dwelling on what other people have done to you, and so on. Given these physical signs (inside the brain and outside), there is no additional mental thing, a 'feeling', needed to form part of it.

You can't have an experience. A fortiori, you can't have a mental experience. There are no mental experiences going on in our heads. What people do are learn secrets, go white-water rafting, solve anagrams, get angry, acquire a taste for armagnac, puzzle over problems, and so on. In the world, things happen, but that doesn't license you to conclude that there's ultimately only one kind of thing, a 'thing', and only one kind of thing it can do, viz 'happen'. Grouping white-water rafting with drinking armagnac and opening letters doesn't usefully classify them. Grouping mental experiences together as 'mental experiences' doesn't either. There's nothing 'mental' in common among so-called mental experiences. There's nothing they have that just corresponds to the 'mental' part: no qualia.

You can solve anagrams silently or by muttering to yourself, writing the letters on paper or revolving a visual image of them, trying every combination in order or waiting for the answer to pop into your head as you walk: other people can watch you doing it, can tell you're doing it by the way your lips are moving, or perhaps can't tell at all if you carefully keep it all in your head. If I steal your wallet I'll try to carefully keep the whole action away from the attention of you, onlookers, and security cameras. Nothing in this is specifically or cardinally 'mental', just more or less overt.

Other people can see me seeing things. My face changes, I become alert, or embarrassed, or relieved, I might even say something. There are some 'mental' acts, typically the most general of them, like seeing, thinking, believing, wondering, that are quite often conducted without much external behaviour. But, no more than counting or solving anagrams, they're not essentially private. There is no essential interior thing required. The brain, blood, heart, nerves, all work in various characteristic ways as we do these things, but none of them requires any inaccessible and unverifiable thing deep inside the brain.

A lot of philosophers like to multiply entities beyond necessity, and a very common production of their reveries is these miniature duplicates of real things, to use instead of the real things: according to fashion they go by various names, ideas, sense impressions, sense data, and today's fashionable word is qualia, but none of them are real, or descriptively useful. They're all just a category mistake.

Written in haste and in brief as a private e-mail reply to Tem42, not intended for noding. Later modestly revised before posting here. I really don’t want to discuss this.